Question 19

[eo5]

Should the answer be B instead of D? Wouldn't Cl- move from Y to X due to chemical gradient?

[admin]

Don't forget: in the instructions of every MCAT, it always says "choose the best answer to the question".

It is true that Cl- ions will diffuse from Y to X. But that is a very incomplete answer compared to D.

The truth is some Cl- ions will diffuse from X to Y. It is less likely for ions to diffuse against a chemical gradient BUT that is the nature of a dynamic equilibrium. Just think of all the chemical reactions that you have written where there were double sided arrows indicating that though you may be looking at the forward reaction, nonetheless both directions are possible.

And there is another element altogether: the electric gradient will have an important impact on the movement of an ion (Cl-). If it were the case that the membrane potential was initially neutral, then the second one Cl- ion crossed over to X then the X side would be net negative meaning there would be a gradient to go from X to Y for any anion (Cl-).

This is why D is definitely the best answer choice.

[sam.philbr2063]

Sorry but I don't see how that makes sense. Since we know from the passage that net ion concentrations are balanced we know that no electrochemical gradient exists.

Suppose that Y is supposed to diffuse to X to balance the chemical gradient, it in turn creates an electrochemical gradient allowing some Cl(-) from X to flow back to Y.

Now --> IF enough Cl flows back to Y so that the electrochemical gradient goes to zero (no longer exists) then we have the exact situation we started with, i.e. a chemical gradient stimulating the movement of Cl from Y to X. Therefore we KNOW that when an electrochemical gradient ceases to exist that there MUST be be a chemical gradient and ion flow will not "stop."

The only option left is that a net amount of Cl ions do in fact flow from Y to X until the equilibrium point where the electrochemical gradient pushing an excess of anions (Cl) from X to Y is equal to the chemical gradient that wants to push an excess of Cl ions from Y to X.

D is an impossible answer since it suggests that ions will move spontaneously at the beginning of the scenario, but will then return to their original position and no longer move spontaneously.

[jellywing_2058]

You are correct about the following:

[quadalpha]

B gives the important information about net direction of ion movement; D is a generic statement ("ions move; no net movement without electrochemical gradient") with no particular relevance to the situation.

[calena7178147]

This answer although “generic” is the best answer.

[dnpgr16513]

Hi,

I understand the answer, however does the protein concentration have no affect with the gradient? I assumed that side X had a more negative charge so K ions would move to the X side. I know this wasn't in the answers but do we not take this into consideration?

thanks so much!

[calena7178147]

The protein does not cross the membrane, but prevents diffusion of cations and favors diffusion of anions (Donnan effect). Therefore, assume K is not moving.

[dwstar183700]

I understand why D is the correct answer, since it is the generic movement of ions, however, I don't understand why B and not C would make sense. Since there is both Cl- and Prot- on X, shouldn't the Cl- be moving from X to Y, since there is a higher concentration in X, therefore Cl- would move from a higher concentration (X) to lower concentration (Y).

Am I not getting this concept right?

Thanks

[hua8986059]

no wonder these questions only 2% or less ppl get right lol.. I thought I knew electrochemical gradients well until I got owned on these 2x questions. I mean if I took time to think it through maybe but spending 5 min each on these questions would not be beneficial to my overall score...

got to learn to think quicker....